This application is based on Japanese Patent Application No. 10-205593 (1998) filed Jul. 21, 1998 and Japanese Patent Application No. 11-200996 (1999) filed Jul. 14, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a printing apparatus having a plurality of print heads to print on a printing material, and more specifically to a printing apparatus that controls print timing among the plurality of the print heads to adjust printing positions by the plurality of printing heads.
2. Description of the Prior Art
One of the known printing apparatus is a color printer of an ink jet type and its main construction is shown in FIG. 1.
When performing printing to a paper 105 on a platen 106, at first, a motor 103 is driven and its driving force is transmitted through a drive belt 109 to a carriage 102 which in turn is moved to where a home position sensor 108 is located. Next, the carriage 102 is moved in a direction of arrow A in the figure to scan over the printing paper. During this scan, color inks of black K, cyan C, magenta M and yellow Y are ejected from print heads 120, 121, 122, 123, respectively, at predetermined timings to print an image. After a predetermined length of the image has been printed, the carriage 102 is stopped and then moved in a direction of arrow B opposite the arrow A and returned to the position of the home position sensor 108. During this return pass, the paper is fed by a distance equal to the width of an array or strip of printed dots of inks printed by the print heads 120-123. That is, paper feed rollers 100, 101 are driven by a paper feed motor 107 to advance the paper in a direction of arrow C in the figure. With the above-described operation repeated, the printing of a color image proceeds. Reference numeral 111 represents a paper-detecting sensor.
When the color printing is performed by using the construction described above, ink droplets of black K, cyan C, magenta M and yellow Y ejected from the print heads 120-123 need to be landed on the paper at each pixel in a predetermined overlapping or an adjacent positional relationship. When, however, the mounting positions of the print heads on the carriage 102 are shifted due to a replacement of the print head or the like, the ejected ink droplets may fail to be landed in the predetermined overlapping or the adjacent positional relationship, deteriorating the print quality. One known method for solving this problem involves printing chart patterns 130-133 for registration deviation detection as shown in FIG. 2 before actual printing and adjusting the printing positions of each head based on the patterns. More specifically, reading the chart patterns is performed by means of a sensor or the like and time differences between detecting respective charts in a signal 134 output from the sensor is measured, so that registration deviations among the print heads are detected. Then, based on the detected deviations, the ejection timings of the respective heads are adjusted. Thereby, the respective color dots are controlled to overlap each other at same positions, for example. In more detail, the reading sensor is mounted on the carriage and is made to scan and read the chart patterns 130-133 so that the respective times T1-T4 are measured as time from a leading edge to a trailing edge of the signal 134 output from the sensor. Then, a median value of each time is calculated to determine differential times t1-t3 between the median values of the adjacent patterns. Further, based on comparison between respective values in the case that the respective heads are in respective proper positions and the measured differential value t1-t3 obtained as described above, positional deviation values are calculated. These calculated values are used for adjusting the ejection timings of the print heads 120-123 so that the actual ink landed positions can be in accord with each other.
In the above prior art example the carriage traveling times are measured and, based on this measurement, the positional deviations among the heads are calculated in the form of time. In this case, if a motor for driving the carriage is that can be controlled at a constant driving speed as with a stepping motor, the positional deviation may be measured to some precision. However, when the constant speed control is relatively difficult, as with a DC motor, variations in the carriage speed will affect the measurement in time of the positional deviations, making the precise measurement of positional deviations impossible. In other words, because of the speed variations, it is not guaranteed that, for the same time period, a distance traveled by the carriage during the measurement of the deviations is equal to the travel distance during the actual printing operation. This means there is an essential problem that the adjustment based on the measured deviations is not reflected on the actual printing.
Further, in another conventional example, it is also possible that the positional deviation is measured based on signals from an encoder that detects the carriage position while traveling and outputs signals as references for the ejection timings of individual heads, instead of based on the travel times of the carriage as in the above case, and the measurement of the positional deviations free from influences of variations in the carriage speed can be realized. With this method, however, because the output signal from the encoder is normally output at intervals corresponding to the intervals of ejection timings of the print heads, there is a drawback that the positional deviations can only be measured with a relatively rough precision with dot intervals as the minimum unit.
An object of the present invention is to provide a printing apparatus capable of correcting printing position deviations among a plurality of heads with high precision while minimizing the influence of variations in the head travel speed.
In the first aspect of the present invention, there is provided a printing apparatus for performing printing on a printing medium by using a plurality of print heads, comprising:
scanning means for causing a plurality of print heads to scan over the printing medium relatively to each other;
print position detection means for detecting a print position of each print head with respect to the printing medium during scanning by the scanning means and for outputting a position detection signal representing the print position thus detected;
pattern printing means for causing the plurality of print heads to print respective registration adjustment patterns during scanning by the scanning means;
pattern detection means for reading the registration adjustment patterns during scanning in the relative scanning direction to output signals representing the print positions of the plurality of the print heads; and
registration adjustment means for determining, for each of the plurality of the print heads, a time difference between a first predetermined timing in pattern printing obtained from the signal output by the pattern detection means and a second predetermined timing obtained, in connection with the first predetermined timing, from the position detection signal output by the print position detection means, the time difference being smaller than a period of the position detection signal, and for correcting respective print timings of the plurality of the print heads according to the time difference between the plurality of the print heads.
In the second aspect of the present invention, there is provided a registration deviation detection method in a printing apparatus using a plurality of print heads to perform printing on a printing medium, comprising the steps of:
preparing print position detection means for detecting a print position of each print head with respect to the printing medium during scanning of the print heads and for outputting a position detection signal representing the print position thus detected, and pattern detection means for reading registration adjustment patterns during scanning in a relative scanning direction to output signals representing the print positions of the plurality of the print heads;
printing the registration adjustment patterns by using the plurality of the print heads while the plurality of the print heads are scanned; and
determining, for each of the plurality of the print heads, a time difference between a first predetermined timing in pattern printing obtained from the signal output by the pattern detection means and a second predetermined timing obtained, in connection with the first predetermined timing, from the position detection signal output by the print position detection means, the time difference being smaller than a period of the position detection signal, and for correcting respective print timings of the plurality of the print heads according to the time difference between the plurality of the print heads.
According to above-stated structure, based on a result of reading the registration adjustment patterns, time differences are calculated between print start timing obtained from these patterns and the leading or trailing edge timing of the output signal for each head, which is detected in connection with the print start timing, from the print position detection means, such as an encoder, that outputs a signal representing stationary coordinates with respect to the apparatus. A difference between these time differences, which are determined for individual print heads, is taken as the print timing deviation of the print heads with respect to each other and then corrected. Thereby, even when there are variations in the print head scanning speed, the effect of these speed variations on the measurement of the print timing deviations can be limited to a small fraction of time (i.e., the time difference described above) which is measured with timing of the leading edge in the output signal from the print position detection means taken as a reference. This in turn reduces the amount of speed variations appearing in the deviation amount being measured. In this case, because the signal output from the print position detection means represents stationary coordinates with respect to the apparatus which are not affected by the speed variations, it is possible to make the reference used for the measurement of the deviation free from influences of the carriage speed variations. Further, the time difference is smaller than a pitch of the print positions corresponding to periods or cycles of the output signal from the print position detection means and therefore it is possible to perform finer corrections.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.